This invention relates to a novel and improved power transmission for tractors and the like, and more particularly to a power transmission in which a fluid operated speed-change mechanism for travelling power is employed.
There is known from U.S. Pat. Nos. 3,231,117, 3,017,941 and 3,831,690, a power transmission which is provided with a fluid operated speed-change mechanism comprising a plurality of speed-change gears which are rotatably mounted on a lay shaft arranged in the transmission path of travelling power from the engine to the final drive means such as wheels and a plurality of fluid actuated clutches each of which is associated with each of the speed-change gears for selectively connecting respective speed-change gears to the lay shaft. The power transmission having such fluid operated speed-change mechanism has an advantage that handling for the change in vehicle speed may be carried out with ease and lightness for the following reasons. The handling is carried out by handling or positioning a change-over valve for controlling the supply of fluid to fluid actuated clutches employed in the fluid operated speed-change mechanism. The speed-change mechanism of fluid actuated type does not have a factor causing some difficulty of speed-changing in a mechanically operated speed-change gearing that engagement between two speed-change gears by shifting one of the gears becomes difficult when such two gears are not faced to each other so that an engagement is achieved as they are. The fluid actuated speed-change mechanism can be handled for speed-changing without disengaging the main clutch arranged immediately behind the engine. The power transmission having a fluid operated speed-change mechanism has a further advantage that it can be made with ease such that, by gradually enlarging fluid pressure applied to fluid actuated clutches therein, no shock is caused when a speed-change is carried out.
The present invention aims to solve a problem involved with power transmissions of this type so that the foregoing advantages can be realized in tractors and the like. Theoretically, when in a tractor or the like having a fluid operated speed-change mechanism, such speed-change mechanism is in the neutral state thereof, namely when all of the fluid actuated clutches in such speed-change mechanism are in the non-operated state so that all of the rotatably mounted speed-change gears on a lay shaft on which the clutches are mounted are made freely rotatable, the power transmission path is cut off at such fluid operated speed-change mechanism so that no transmission of power ought to be caused from the engine under operation to the final drive means or from the final drive means toward the engine. In practice, however, it is often experienced that one and another friction elements of the fluid actuated clutches which elements have been released from forced engagement by fluid pressure may become engaged with some force of engagement by torque transmitted to the fluid operated speed-change mechanism from the engine or from the final drive means so that an unexpected transmission of power is caused through the fluid operated speed-change mechanism under the neutral or non-operation state of such mechanism. This may cause a dangerous unexpected start of the vehicle.
Accordingly, a primary object of the present invention is to provide a novel power transmission for tractors and the like having a fluid operated speed-change mechanism which transmission is fashioned such that the aforementioned unexpected transmission of power under the neutral state of the fluid operated speed-change mechanism is prevented with certainty.
Another object of the present invention is to provide a power transmission for tractors and the like in which such prevention of unexpected transmission of power is achieved without any particular handling.
Still another object of the present invention is to provide a novel power transmission for tractors and the like which attains such prevention of unexpected transmission of power with a simple construction.
A further object of the present invention is to provide a power transmission for tractors and the like in which lay shaft mounting fluid actuated clutches of the fluid operated speed-change mechanism is braked under the neutral state of the speed-change mechanism thereby mentioned unexpected transmission of power being prevented and in which the braking of the lay shaft is rapidly released, when the fluid operated speed-change mechanism is exchanged or turned from the neutral state to the operation state, so that there can be avoided the problem of the lay shaft becoming connected to the power source or engine under a condition that the shaft is still braked, whereas, when the fluid operated speed-change mechanism is exchanged or turned from the operation state to the neutral state, such lay shaft becomes gradually braked so that a shock which may be caused by a rapid braking does not result.
A still further object of the present invention is to provide a novel power transmission for tractors and the like which, in a case when a mechanically operated speed-change mechanism is further provided in the transmission path of travelling power in series to and at behind the fluid operated speed-change mechanism, permits an easy handling of such mechanically operated speed-change mechanism for changing the speed-change ratio notwithstanding the provision of a mechanism which prevents unexpected transmission of power through the fluid operated speed-change mechanism under the neutral state thereof.